Gravity-feed tunnel kiln



1 Jan. 13, 1931. T. G. McDou v 1,783,885

GRAVITY FEED TUNNEL KILN i Filed June s. 1928 4 Sheets-Sheet 1 Jan. 13, 1931. T. G. MCDOUGALI 5 GRAVITY FEED TUNNEL KILN Fild June 8, 1928 4 SheetsSheet 2 I! II I 1 3717 I amnion Jan. 13, 1931. T. GQMQDQUGAL v 1,788,885

GRAVITY FEED TUNNEL KILN Filed June a, 1928 I 4 Sheets-Sheet 3 autumn,

Jan. 13, 1931. T. GuMQDOUGAL 1,788,885

GRAVITY FEED TUNNEL KILN Filed June 8. 1 9 28 i'sheets-sheet 4 Patented Jan. 13, 1931 UNITED STATE PATENT OFFICE' TAINE G. MCDO'UGAL, OF FLINT, MICHIGAN, ASSIGNOR TO A C SPARK PLUG COMPANY,

OF FLINT, MICHIGAN, A COMIEANY OF MICHIGAN GBAVITY-FEEIlJ TUNNEL KILN Application filed June 8, 1925. I Serial no. 283,836.

This invention relates to tunnel kilns adapted for the burning of ceramic wares, although in some of its aspects, it is of use in the heat treating of Various articles. It is characterized by the arrangement of the kiln in such manner that the force of gravity is available to effect the passage of the wares through it. By thus dispensing with mechanical conveyors, it is possible to employ a 7 o kiln of full tunnel form so that there are 110 slots or sliding joints to be sealed against escape of heat. The wares may be passed through the tunnel'without the interposition of a support of any sort, but wheretheir shape does not permit of this, refractory supports may be employed. These supports need be of justsufiicient mass to carry the wares and as a consequence, the amount of heat absorbed by them will be very small. This small heat absorption is to be contrasted with the very great amount of heat absorbed by the large masses of refractories required in previous constructions to protect the metal parts of the conveyor from the heat of the firing chamber. As a consequence, the efliciency of my improved kiln will be very much higher than that of previous designs. Since less heat is absorbed by the carriers the time required for burning the wares will be considerably.

3o reduced so that the apparatus will handle a greater amount of wares in a given time. The construction of my improved kiln and the method of supporting the wares in their passage through the kiln is also such as to insure uniform heating, thus reducing the number of fractures to a minimum.

My invention is susceptible of considerable modification in practice as will be pointed out in the course of the following description.

Figures 1, 2 and 3 when placed together Figure 12 is an enlarged section showing a'spark plug insulator mounted in one of the carr ers in place in a guideway in the kiln.

Figure 13 is a top plan view of the carriers.

Figure -14 is a section through the firing- Figure' 16 is a sectional 'view through the firing zone of a tunnel kiln of substantially .the same construction as that shown in Figures 14 and 15, but showing the tile mounted on special carriers.

' Figure 17 is a perspective view of one of the carriers of Figure 16.

Referrin first to Figures 1 to 4, the tunnel kiln there i lustrated comprises a preheating zone, a firing zone, and a cooling zone. The preheating zone is illustrated in Figure 1, and its extent is indicated by the bracket A. The firing zone is illustrated in Figure 2, and its extent is indicated by the bracket B. The cooling zone is illustrated in Figure 3, and its extent is indicated by the bracket G. The tunnel is provided throughout its length with a guideway 10 for receiving the wares,

or, in the case of spark plug porcelains 12 or similar small articles, for receiving the carriers 14 in which the articles in turn are supported. The guideway is preferably formed by notching out the upper adjacent .r1ers.

The kiln is so arranged that the wares,

or the wares and their carriers, travel along the guideway by gravity action. This is accomplished by building the kiln so that it extends ,at an inclination as by supporting the kiln upon steel plates 18 extending between angle irons 22 which, in turn, are

supported by spaced trestle-like structures 24, formed out of structural iron.- The trestles 24'are of graduated height so as to give the kiln the desired inclination.

I have shown the guideway 10 as extending beyond the discharge end 26 of the tunnel. A conveyor 28 preferably in the form .15 may be modified as shown in Figure 16 and the tile or the wares may be supported of spaced belts 30 passing over suitable pulon suitable carriers such as shown at 42 in leys 32 is arranged to receive the wares distFigure 17. Where the carriers are employed,

charged from the tunnel. In the case of such articles as spark plug insulators, the projecting ends of the wares are received in the space between the belts. Obviously, the belts ma be made of any suitable material, but owing to'the heated condition of the articles When discharged from the tunnel, I prefer to use flexible steel belting.

The angle of inclination of the kiln is preferably such that'the articles slide through the tunnel under the action of gravity, and to control the rate of movement any suitable means may be provided. In the construction illustrated, the inertia of the mass of wares on the conveyor 30 is sufiicient to prevent any rapid sliding movement, and by driving the conveyor at very slow speed, the ware is permitted continuous movement through the tunnel at the same slow rate. If preferred, positive braking or indexing means may be provided at the discharge end of the tunnel, or may be incorporated in the conveyor to positively control the discharge of the wares and prevent anypossibility of rapid sliding movement. By controlling the speed of passage of the wares through the tunnel, the firing operation is obviously controlled so that the wares are preheated, fired and cooled at the desired rate. Should it be desired in the case of any particular wares.

to provide different speeds of travel throughout the different zones, this may be accomplished by the provision of suitable indexing means between the zones but the simpler way of accomplishing the same result is to vary the lengths of the zones in accordance with the proportion of the total time in which the specified operation is to continue.

The manner of passing the wares through the kiln should now be-apparent. The steps in firing will now be described.

Figure 8 is a cross section of the firing zone as constructed for the burning of spark plug insulators. Burning is accomplished .by supplying to the tunnel at this point pre- 'of the tunnel above the wares, since it is merely the glost coating 40 on the top of the tile that is being burned. For glost burning as well as for the burning of the tile itself,

the tile are supported in horizontal position, as shown in Figure 16 and this insures an even coating of the glaze. In burning the tile itself, it will be necessary to emplo carriers 42 having their wares supporting portions 44 sufficiently closely spaced so that the span of tile will not be so great as to cause it to sag and become deformed. It will be noted also that in Figure 16, the flue 38' through which the combustible is supplied to the firing zone is located near the top of the tunnel so that the hot gases are projected directly upon the tile.

The burned gases from the firing chamber pass upwardly through the preheating zone and heat the incoming wares. To prevent too rapid escape of the gases as a result of'draft produced by the inclined position of the kiln, a baffle 45 may be provided between the firing zone and the preheating zone, and other bafiies such as indicated at 46 in Figure 1: may be provided along the length of the pre-. heating zone. However, to insure that the preheating of the ware will be as uniform as possible, I prefer to employ as few obstructions as possible in the roof of the kiln. Upon leaving the firing zone, the wares pass.

through the portion 48 of the tunnel, which,

as shown in Figure 9, is of restricted cross section conforming closely to the shape of the wares and carriers. This portion acts as abafile between the firing zone and the cooling zone C. The cooling zone is of known design, and is arranged to efiect gradual lowering of the temperature of the ware. Thus the upper portion of the cooling Zone is arranged for i direct air cooling by the provision of fiues 50 surrounding the tunnel and adapted for the passage of air which is discharged through pipe 52 leading to a suitable stack. From the air cooled region the wares pass 'into the portion of the tunnel, best. shown in Figure 11, through which run pipes 54 containing water or other cooling medium. This is the final cooling and the wares are then'discharged 011 to the conveyor 28 previously described.

The various steps in the heating and cool-, ing of the wares in their passage through the tunnel may be varied as desired. Thus the firing portion of the kiln may be of the muffle type or if preferred, the wares may be heated by electricity by mounting suitable resistors in the walls of the kiln. The preheating zone may also be of the mufile type and obviously the cooling of the wares may be accomplished in varied ways.

Not only does my improved kiln provide a very simple way of passing the wares through the kiln, but it is also much more meaasz .eficient from the heating standpoint than prior constructions. In previous tunnel kilns, it has been customary to employ conveyors of various sorts to move the wares through the kiln. These conveyors took the form of cars covered with a great thicknesses of refractory to protect the. running gear dispensed with and as aconsequence, the tunnel may be more simplyconstructed since no slot need be provided to'receive the conveyor. The Wares are supported upon refractory carriers as shown in Figures 12 and 16.

These may be made of .the minimum size necessary to support the wares, and as a consequence absorb but a fraction of the amount of heat which would be absorbed by the large mass of refractory needed to protect the metallic parts of the conveyors used in prior constructions. The carriers may be a permanent part ofthe equipment'of the kiln, and in such a case shouldbe constructed of high grade refractory, such as fine grained alundum. Obviously, in glost burning such as is illustrated in Figures 14 and 15 where no carriers are employed the maximum efficiency is also obtainable. The kiln illustrated is of the single file type wherein the wares are exposed to the hot gases and to radiation from the walls of the kiln on substantially all sides so that even and rapid heat absorption is assured.

lnsome cases it may be desirable to sup port the wares in some other manner than those illustrated. 'For instance, in the case of spark plug porcelains they may be mounted upon upright refractory pins projecting from the carriers. However, the illustrated arrangement issatisfactory and with the fuel mixture supplied both above and below the carriers as in the construction shown in Figures 2 and 8, and with carriers of small mass,

. the shielding effect of the carriers onthe central portion of the insulator will not be objectionable'. I

Iclaim:

1. An inclined heat treating tunnel having its entrance end at a higher level than its discharge end, and a driven conveyor at the discharge end of the tunnel for receiving wares therefrom, the mass of the wares on the conveyor resisting sliding movement of the wares throu h the tunnel, and the rate of movement 0? the wares being determined by the speed at which the conveyor is driven.

2. A carrier for supporting a spark plug insulator during firing, consisting of a plate of ceramic material apertured to' engage the enlarged portion of the plug.

3. A heat treating tunnel having itsentrance end at higher level than its discharge end, means in said tunnel for supporting the wares in their passage therethrough by gravity action, and the means for controlling the rate of travel of the wares through the tunnel, said last named means comprising a con- .veyor-receiving the burned wares.

' 4. A tunnel kiln provided with a firing chamber I having vertically spaced heating means, and'a guidewayfor supporting wares in their passage through the tunnel, and extending longitudinally of the tunnel between said heating means, said. heating means include conduits for supplying combustible to the firing chamber above and below said guideway.

5. A tunnel kiln for burning ceramic wares, said kiln being tubular in cross section and having refractory supports extending inwardly from opposite walls, refractory members resting on said supports and straddling the space therebetween, said members being adapted to carry articles to be burned.

6. A tunnel kiln for burning ceramicv wares, including a firing chamber comprising a guideway wholly inclosed in the tunnel, supports for the wares mounted on said guideway and conduits for supplying combustible to the tunnel communicating therewith above and below the guideway.

7. A tunnel kiln for burning ceramic wares, said kiln including a firing zone tubular in cross section and having refractory supports extending inwardly from opposite walls thereof and spaced above the bottom of the kiln, wares supported by said supports and having portions exposed above and below the supports, and means for applying heat to the kiln above and below said supports to effect complete burning of the wares.

8; A tunnel kiln for burning ceramic wares, said kiln including a firing zone tubular in cross section and having refractory supports la a extending inwardly from opposite walls thereof and spaced above the bottom of the kiln, wares supportedby said supports and having portions exposed above and below the supports, and means for-applying heat to the kiln above and below said'supports to effect complete burning of the wares, said supports being apertured to receive ceramic wares to be burned.

Intestimonywhereof I aflix my signature. 1

TAINE G. McDOUGAL. 

